1. Field of the Invention
This invention relates to a mutilayered optical information-recording medium (hereafter, optical disk) that has two or more information-recording surfaces in the thickness direction of a light transmitting substrate and a process for the manufacture thereof
2. Description of the Prior Art
Optical disks which record optically readable information and allow reading of the recorded information using a laser beam spot have heretofore been available. Optical disks including compact disks (CDs) and CD-ROMs have become widespread in use, particularly over the recent years.
CD-ROMs recently have come to be used not only in computers but also in multimedia game CD-ROMs and are increasingly replacing magnetic disks (floppy disks) and ROM cartridges both in computer and game applications. Furthermore, a high density CD version called the DVD (digital videodisk) is about to enter the field of movies and multimedia.
Recently proposals have been made of multilayered optical disks that enable the recording of massive quantities of information. In contrast to the conventional CD that has a single-layer information-recording surface in which information signals are recorded only on one surface thereof on a substrate, the mutilayered optical disks are structured to have a multiple number of information-recording surfaces in the thickness direction of the substrate.
Referring to FIG. 1, the aforementioned multilayered optical disk is now described below.
FIG. 1 is a drawing illustrating the structure of a conventional multilayered optical disk. FIG. 1 shows a partial cross-sectional view of the optical disk in its tracking direction.
As illustrated in FIG. 1, the conventional multilayered optical disk 101 has a first reflecting layer 105, a transparent layer 106, a second reflecting layer 107, and a protective layer 108 laminated together in this sequence onto a light transmitting substrate 104.
Crenulated pits 104A corresponding to information are generated on the substrate 104, and pits 104B corresponding to information are generated on the above transparent layer 106. In other words, the optical disk 101 has two information-recording surfaces in the thickness direction of the substrate 104, where the surface on which pits 104A are formed on the light transmitting substrate 104 is the first information-recording surface 102, and the surface on which pits 104B are formed on the light transmitting substrate 106 is the second information-recording surface 103.
The first reflecting layer 105 which is formed between the first information-recording surface 102 and the second information-recording surface 103 is made of a material with a certain degree of light transmittance to enable light to be incident on the second information-recording surface 103 or to be reflected.
If three or more information-recording surfaces are to be formed, a second transparent layer is formed on the second reflecting layer 107 on the above transparent layer 106, followed by forming the pits on the second transparent layer, generating a third recording surface, and a similar procedure is used to form a fourth information-recording surface and beyond.
Incident playback laser beam for reading information recorded on each of the above information-recording surfaces (102, 103) is directed through the underside of the above substrate 104. The playback light beam pickup is equipped to accurately recognize the spacing between adjacent information-recording surfaces, which may be as narrow as several tens of microns (.mu.m), and to be focused onto the desired information-recording surface, so as to read out the information present on each information-recording surface.
As described above, the multilayered optical disk which is structured to have two or more information-recording surfaces in the thickness direction of substrate 104 can record a greater amount of information than can a conventional single layer structured optical disk.
Referring to FIG. 2, a conventional process for the manufacture of optical disk 101 having the above construction is described as follows.
First, as illustrated in FIG. 2(A), a stamper (not illustrated) with a pattern negative to pits 104 A on the first information-recording surface is used to mold, for example, by injection molding, an optical disk substrate 104 having pits 104A on its surface.
Then, as illustrated in FIG. 2(B), a first reflecting layer 105 is formed on the surface of substrate 104 having pits 104A formed thereon by a film-forming method such as sputtering, vacuum vapor depositing, spin coating or the like. It should be recalled that as mentioned above, the first reflecting layer 105 is a reflecting film that has a certain degree of light transmittance.
Then, a second information-recording surface is formed on top of the above first reflecting layer 105. The second information-recording surface is formed by the process well-known in the art as "the 2P process."
That is, as illustrated in FIG. 2(C), a stamper 141 with crenulated 141A pattern negative to pits 104B of the second information-recording surface is used, a UV-curable resin 142 is deposited as a droplet onto the first reflecting layer 105 of the above substrate 104, and then the stamper 141 is pressed at its signal-bearing surface against the substrate 104 having the UV-curable resin 142 deposited thereon so as to spread the UV-curable resin 142 to a uniform thickness.
Incidentally a nozzle (not illustrated) is used to suction off any resin that has overflown from the outer peripheral of substrate 104 as the stamper 141 is pressed.
Then, when the above UV-curable resin 142 is cured by irradiating UV from the substrate 104 side, followed by peeling off the stamper 141, an optical disk substrate is obtained on which is generated pits 104B that provide a second information-recording surface 103, as illustrated in FIG. 2(D).
Lastly, a second reflecting layer 107 is formed as a film of aluminum, gold, or the like, by sputtering or by vacuum film formation such as vacuum vapor deposition or the like, followed by forming a protective film 108 onto the second reflecting layer 107 and print a label (not illustrated) on the protective layer 108, thereby completing the formation of a multilayered optical disk as illustrated in FIG. 2(E).
It should be noted that the playback principle imposed on a multilayered optical disk as described above requires that the spacing between adjacent information-recording surfaces be under strict control. For example, a 40 .mu.m thick film spacing between adjacent information-recording surfaces allows a variation in thickness of only about .+-.2 .mu.m.
However, "the 2P process" mentioned above even with improved precision could achieve at best a level of .+-.5 .mu.m. In addition, "the 2P process" is inferior in productivity relative to that of injection molding, requiring as long as 2 minutes per surface for molding a second and subsequent information-recording surfaces.
It may be possible to increase in practice the molding rate of "the 2P process", but that would lead to problems such as reduced film thickness precision, trapping of air bubbles, unsatisfactory suction-removal of the resin overflow, and the like.
That is, manufacture of the above multilayered optical disk by a conventional manufacturing process would be plagued with problems in terms of both precision and productivity.